System and methods for determining nerve proximity, direction, and pathology during surgery

ABSTRACT

The present invention involves systems and methods for determining nerve proximity, nerve direction, and pathology relative to a surgical instrument based on an identified relationship between neuromuscular responses and the stimulation signal that caused the neuromuscular responses.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/080,493 filed by Gharib et al. on Apr. 5, 2011 (the contents beingincorporated herein by reference) which is a division of U.S. patentapplication Ser. No. 12/711,937 filed by Gharib et al. on Feb. 24, 2010(the contents being incorporated herein by reference) which is acontinuation of U.S. patent application Ser. No. 10/754,899 filed byGharib et al. on Jan. 9, 2004 (the contents being incorporated herein byreference), which is a continuation of PCT Patent Application Ser. No.PCT/US02/22247 filed on Jul. 11, 2002 and published as WO03/005887 (thecontents being incorporated herein by reference).

BACKGROUND OF THE INVENTION

I. Field of the Invention

This invention relates to nerve monitoring systems and to nerve musclemonitoring systems, and more particularly to systems and methods fordetermining nerve proximity, nerve direction, and pathology duringsurgery.

II. Description of Related Art

Systems and methods exist for monitoring nerves and nerve muscles. Onesuch system determines when a needle is approaching a nerve. The systemapplies a current to the needle to evoke a muscular response. Themuscular response is visually monitored, typically as a shake or“twitch.” When such a muscular response is observed by the user, theneedle is considered to be near the nerve coupled to the responsivemuscle. These systems require the user to observe the muscular response(to determine that the needle has approached the nerve). This may bedifficult depending on the competing tasks of the user. In addition,when general anesthesia is used during a procedure, muscular responsemay be suppressed, limiting the ability of a user to detect theresponse.

While generally effective (although crude) in determining nerveproximity, such existing systems are incapable of determining thedirection of the nerve to the needle or instrument passing throughtissue or passing by the nerves. This can be disadvantageous in that,while the surgeon may appreciate that a nerve is in the generalproximity of the instrument, the inability to determine the direction ofthe nerve relative to the instrument can lead to guess work by thesurgeon in advancing the instrument and thereby raise the specter ofinadvertent contact with, and possible damage to, the nerve.

Another nerve-related issue in existing surgical applications involvesthe use of nerve retractors. A typical nerve retractor serves to pull orotherwise maintain the nerve outside the area of surgery, therebyprotecting the nerve from inadvertent damage or contact by the “active”instrumentation used to perform the actual surgery. While generallyadvantageous in protecting the nerve, it has been observed that suchretraction can cause nerve function to become impaired or otherwisepathologic over time due to the retraction. In certain surgicalapplications, such as spinal surgery, it is not possible to determine ifsuch retraction is hurting or damaging the retracted nerve until afterthe surgery (generally referred to as a change in “nerve health” or“nerve status”). There are also no known techniques or systems forassessing whether a given procedure is having a beneficial effect on anerve or nerve root known to be pathologic (that is, impaired orotherwise unhealthy).

Based on the foregoing, a need exists for a better system and methodthat can determine the proximity of a surgical instrument (including butnot limited to a needle, catheter, cannula, probe, or any other devicecapable of traversing through tissue or passing near nerves or nervestructures) to a nerve or group of nerves during surgery. A need alsoexists for a system and method for determining the direction of thenerve relative to the surgical instrument. A still further need existsfor a manner of monitoring nerve health or status during surgicalprocedures.

The present invention is directed at eliminating, or at least reducingthe effects of, the above-described problems with the prior art, as wellas addressing the above-identified needs.

SUMMARY OF THE INVENTION

The present invention includes a system and related methods fordetermining nerve proximity and nerve direction to surgical instrumentsemployed in accessing a surgical target site, as well as monitoring thestatus or health (pathology) of a nerve or nerve root during surgicalprocedures.

According to a broad aspect, the present invention includes a surgicalsystem, comprising a control unit and a surgical instrument. The controlunit has at least one of computer programming software, firmware andhardware capable of delivering a stimulation signal, receiving andprocessing neuromuscular responses due to the stimulation signal, andidentifying a relationship between the neuromuscular response and thestimulation signal. The surgical instrument has at least one stimulationelectrode electrically coupled to said control unit for transmitting thestimulation signal, wherein said control unit is capable of determiningat least one of nerve proximity, nerve direction, and nerve pathologyrelative to the surgical instrument based on the identified relationshipbetween the neuromuscular response and the stimulation signal.

In a further embodiment of the surgical system of the present invention,the control unit is further equipped to communicate at least one ofalpha-numeric and graphical information to a user regarding at least oneof nerve proximity, nerve direction, and nerve pathology.

In a further embodiment of the surgical system of the present invention,the surgical instrument may comprise at least one of a device formaintaining contact with a nerve during surgery, a device for accessinga surgical target site, and a device for testing screw placementintegrity.

In a further embodiment of the surgical system of the present invention,the surgical instrument comprises a nerve root retractor and wherein thecontrol unit determines nerve pathology based on the identifiedrelationship between the neuromuscular response and the stimulationsignal.

In a further embodiment of the surgical system of the present invention,the surgical instrument comprises a dilating instrument and wherein thecontrol unit determines at least one of proximity and direction betweena nerve and the instrument based on the identified relationship betweenthe neuromuscular response and the stimulation signal.

In a further embodiment of the surgical system of the present invention,the dilating instrument comprises at least one of a K-wire, anobturator, a dilating cannula, and a working cannula.

In a further embodiment of the surgical system of the present invention,the surgical instrument comprises a screw test probe and wherein thecontrol unit determines the proximity between the screw test probe andan exiting spinal nerve root to assess whether a medial wall of apedicle has been breached by at least one of hole formation and screwplacement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a surgical system 10 capable ofdetermining, among other things, nerve proximity, direction, andpathology according to one aspect of the present invention;

FIG. 2 is a block diagram of the surgical system 10 shown in FIG. 1;

FIG. 3 is a graph illustrating a plot of the neuromuscular response(EMG) of a given myotome over time based on a current stimulation pulse(similar to that shown in FIG. 4) applied to a nerve bundle coupled tothe given myotome;

FIG. 4 is a graph illustrating a plot of a stimulation current pulsecapable of producing a neuromuscular response (EMG) of the type shown inFIG. 3;

FIG. 5 is a graph illustrating a plot of peak-to-peak voltage (Vpp) foreach given stimulation current level (I_(Stim)) forming a stimulationcurrent pulse train according to the present invention (otherwise knownas a “recruitment curve”);

FIG. 6 is a graph illustrating a plot of a neuromuscular response (EMG)over time (in response to a stimulus current pulse) showing the mannerin which maximum voltage (V_(Max)) and minimum voltage (V_(Min)) occurat times T1 and T2, respectively;

FIG. 7 is an exemplary touch-screen display according to the presentinvention, capable of communicating a host of alpha-numeric and/orgraphical information to a user and receiving information and/orinstructions from the user during the operation of the surgical system10 of FIG. 1;

FIGS. 8A-8E are graphs illustrating a rapid current threshold-huntingalgorithm according to one embodiment of the present invention;

FIG. 9 is a series of graphs illustrating a multi-channel rapid currentthreshold-hunting algorithm according to one embodiment of the presentinvention;

FIG. 10 is a graph illustrating a T1, T2 artifact rejection techniqueaccording to one embodiment of the present invention via the use ofhistograms;

FIG. 11 is a graph illustrating the proportion of stimulations versusthe number of stimulations employed in the T1, T2 artifact rejectiontechnique according to the present invention;

FIG. 12 is an illustrating (graphical and schematic) of a method ofautomatically determining the maximum frequency (F_(Max)) of thestimulation current pulses according to one embodiment of the presentinvention;

FIG. 13 is a graph illustrating a method of determining the direction ofa nerve (denoted as an “octagon”) relative to an instrument having four(4) orthogonally disposed stimulation electrodes (denoted by the“circles”) according to one embodiment of the present invention;

FIG. 14 is a graph illustrating recruitment curves for a generallyhealthy nerve (denoted “A”) and a generally unhealthy nerve (denoted“B”) according to the nerve pathology determination method of thepresent invention;

FIG. 15 is flow chart illustrating an alternate method of determiningthe hanging point of a recruitment curve according to an embodiment ofthe present invention; and

FIG. 16 is a graph illustrating a simulated recruitment curve generatedby a “virtual patient” device and method according to the presentinvention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure. The systems disclosed herein boast a variety ofinventive features and components that warrant patent protection, bothindividually and in combination.

FIG. 1 illustrates, by way of example only, a surgical system 10 capableof employing the nerve proximity, nerve direction, and nerve pathologyassessments according to the present invention. As will be explained ingreater detail below, the surgical system 10 is capable of providingsafe and reproducible access to any number of surgical target sites, andwell as monitoring changes in nerve pathology (health or status) duringsurgical procedures. It is expressly noted that, although describedherein largely in terms of use in spinal surgery, the surgical system 10and related methods of the present invention are suitable for use in anynumber of additional surgical procedures wherein tissue havingsignificant neural structures must be passed through (or near) in orderto establish an operative corridor, or where neural structures areretracted.

The surgical system 10 includes a control unit 12, a patient module 14,an EMG harness 16 and return electrode 18 coupled to the patient module14, and a host of surgical accessories 20 capable of being coupled tothe patient module 14 via one or more accessory cables 22. The surgicalaccessories 20 may include, but are not necessarily limited to, surgicalaccess components (such as a K-wire 24, one or more dilating cannula 26,and a working cannula 28), neural pathology monitoring devices (such asnerve root retractor 30), and devices for performing pedicle screw test(such as screw test probe 32). A block diagram of the surgical system 10is shown in FIG. 2, the operation of which is readily apparent in viewof the following description.

The control unit 12 includes a touch screen display 36 and a base 38.The touch screen display 36 is preferably equipped with a graphical userinterface (GUI) capable of communicating information to the user andreceiving instructions from the user. The base 38 contains computerhardware and software that commands the stimulation sources, receivesdigitized signals and other information from the patient module 14, andprocesses the EMG responses to extract characteristic information foreach muscle group, and displays the processed data to the operator viathe display 36. The primary functions of the software within the controlunit 12 include receiving user commands via the touch screen display 36,activating stimulation in the requested mode (nerve proximity, nervedetection, nerve pathology, screw test), processing signal dataaccording to defined algorithms (described below), displaying receivedparameters and processed data, and monitoring system status and reportfault conditions.

The patient module 14 is connected via a serial cable 40 to the controlunit 12, and contains the electrical connections to all electrodes,signal conditioning circuitry, stimulator drive and steering circuitry,and a digital communications interface to the control unit 12. In use,the control unit 12 is situated outside but close to the surgical field(such as on a cart adjacent the operating table) such that the display36 is directed towards the surgeon for easy visualization. The patientmodule 14 should be located between the patient's legs, or may beaffixed to the end of the operating table at mid-leg level using abedrail clamp. The position selected should be such that the EMG leadscan reach their farthest desired location without tension during thesurgical procedure.

In a significant aspect of the present invention, the informationdisplayed to the user on display 36 may include, but is not necessarilylimited to, alpha-numeric and/or graphical information regarding nerveproximity, nerve direction, nerve pathology, stimulation level,myotome/EMG levels, screw testing, advance or hold instructions, and theinstrument in use. In one embodiment (set forth by way of example only)the display includes the following components as set forth in Table 1:

TABLE 1 Screen Component Description Menu/Status Bar The mode label mayinclude the surgical accessory attached, such as the surgical accesscomponents (K-Wire, Dilating Cannula, Working Cannula), nerve pathologymonitoring device (Nerve Root Retractor), and/or screw test device(Screw Test Probe) depending on which is attached. Spine Image An imageof a human body/skeleton showing the electrode placement on the body,with labeled channel number tabs on each side (1-4 on left and right).Left and Right labels will show the patient orientation. The Channelnumber tabs may be highlighted or colored depending on the specificfunction being performed. Display Area Shows procedure-specificinformation. Myotome & Level A label to indicate the Myotome name andcorresponding Spinal Level(s) Names associated with the channel ofinterest. Advance/Hold When in the Detection mode, an indication of“Advance” will show when it is safe to move the cannula forward (such aswhen the minimum stimulation current threshold I_(Thresh) (describedbelow) is greater than a predetermined value, indicating a safe distanceto the nerve) and “Hold” will show when it is unsafe to advance thecannula (such as when the minimum stimulation current thresholdI_(Thresh) (described below) is less than a predetermined value,indicating that the nerve is relatively close to the cannula) and duringproximity calculations. Function Indicates which function is currentlyactive (Direction, Detection, Pathology Monitoring, Screw Test). DilatorIn Use A colored circle to indicate the inner diameter of the cannula,with the numeric size. If cannula is detached, no indicator isdisplayed.

The surgical system 10 accomplishes safe and reproducible access to asurgical target site by detecting the existence of (and optionally thedistance and/or direction to) neural structures before, during, andafter the establishment of an operative corridor through (or near) anyof a variety of tissues having such neural structures which, ifcontacted or impinged, may otherwise result in neural impairment for thepatient. The surgical system 10 does so by electrically stimulatingnerves via one or more stimulation electrodes at the distal end of thesurgical access components 24-28 while monitoring the EMG responses ofthe muscle groups innervated by the nerves. In a preferred embodiment,this is accomplished via 8 pairs of EMG electrodes 34 placed on the skinover the major muscle groups on the legs (four per side), an anodeelectrode 35 providing a return path for the stimulation current, and acommon electrode 37 providing a ground reference to pre-amplifiers inthe patient module 14. By way of example, the placement of EMGelectrodes 34 may be undertaken according to the manner shown in Table 2below for spinal surgery:

TABLE 2 Channel Spinal Color ID Myotome Nerve Level Red Right 1 RightVastus Medialis Femoral L2, L3, L4 Orange Right 2 Right TibialisAnterior Peroneal L4, L5 Yellow Right 3 Right Biceps Femoris Sciatic L5,S1, S2 Green Right 4 Right Gastroc. Medial Post Tibialis S1, S2 BlueLeft 1 Left Vastus Medialis Femoral L2, L3, L4 Violet Left 2 LeftTibialis Anterior Peroneal L4, L5 Gray Left 3 Left Biceps FemorisSciatic L5, S1, S2 White Left 4 Left Gastroc. Medial Post Tibialis S1,S2

Although not shown, it will be appreciated that any of a variety ofelectrodes can be employed, including but not limited to needleelectrodes. The EMG responses provide a quantitative measure of thenerve depolarization caused by the electrical stimulus. Analysis of theEMG responses is then used to determine the proximity and direction ofthe nerve to the stimulation electrode, as will be described withparticularity below.

The surgical access components 24-28 are designed to bluntly dissect thetissue between the patient's skin and the surgical target site. Aninitial dilating cannula 26 is advanced towards the target site,preferably after having been aligned using any number of commerciallyavailable surgical guide frames. An obturator (not shown) may beincluded inside the initial dilator 26 and may similarly be equippedwith one or more stimulating electrodes. Once the proper location isachieved, the obturator (not shown) may be removed and the K-wire 24inserted down the center of the initial dilating cannula 26 and dockedto the given surgical target site, such as the annulus of anintervertebral disc. Cannulae of increasing diameter are then guidedover the previously installed cannula 26 until the desired lumen isinstalled. By way of example only, the dilating cannulae 26 may range indiameter from 6 mm to 30 mm. In one embodiment, each cannula 26 has fourorthogonal stimulating electrodes at the tip to allow detection anddirection evaluation, as will be described below. The working cannula 28is installed over the last dilating cannula 26 and then all the dilatingcannulae 26 are removed from inside the inner lumen of the workingcannula 28 to establish the operative corridor therethrough. Astimulator driver 42 is provided to electrically couple the particularsurgical access component 24-28 to the patient module 14 (via accessorycable 22). In a preferred embodiment, the stimulator driver 42 includesone or more buttons for selectively activating the stimulation currentand/or directing it to a particular surgical access component.

The surgical system 10 accomplishes neural pathology monitoring byelectrically stimulating a retracted nerve root via one or morestimulation electrodes at the distal end of the nerve root retractor 30while monitoring the EMG responses of the muscle group innervated by theparticular nerve. The EMG responses provide a quantitative measure ofthe nerve depolarization caused by the electrical stimulus. Analysis ofthe EMG responses may then be used to assess the degree to whichretraction of a nerve or neural structure affects the nerve functionover time, as will be described with greater particularity below. Oneadvantage of such monitoring, by way of example only, is that theconduction of the nerve may be monitored during the procedure todetermine whether the neurophysiology and/or function of the nervechanges (for the better or worse) as the result of the particularsurgical procedure. For example, it may be observed that the nerveconduction increases as the result of the operation, indicating that thepreviously inhibited nerve has been positively affected by theoperation. The nerve root retractor 30 may comprise any number ofsuitable devices capable of maintaining contact with a nerve or nerveroot. The nerve root retractor 30 may be dimensioned in any number ofdifferent fashions, including having a generally curved distal region(shown as a side view in FIG. 1 to illustrate the concave region wherethe nerve will be positioned while retracted), and of sufficientdimension (width and/or length) and rigidity to maintain the retractednerve in a desired position during surgery. The nerve root retractor 30may also be equipped with a handle 31 having one or more buttons forselectively applying the electrical stimulation to the stimulationelectrode(s) at the end of the nerve root retractor 30. In oneembodiment, the nerve root retractor 30 is disposable and the handle 31is reusable and autoclavable.

The surgical system 10 can also be employed to perform screw testassessments via the use of screw test probe 32. The screw test probe 32is used to test the integrity of pedicle holes (after formation) and/orscrews (after introduction). The screw test probe 32 includes a handle44 and a probe member 46 having a generally ball-tipped end 48. Thehandle 44 may be equipped with one or more buttons for selectivelyapplying the electrical stimulation to the ball-tipped end 48 at the endof the probe member 46. The ball tip 48 of the screw test probe 32 isplaced in the screw hole prior to screw insertion or placed on theinstalled screw head. If the pedicle wall has been breached by the screwor tap, the stimulation current will pass through to the adjacent nerveroots and they will depolarize at a lower stimulation current.

Upon pressing the button on the screw test handle 44, the software willexecute an algorithm that results in all channel tabs being color-codedto indicate the detection status of the corresponding nerve. The channelwith the “worst” (lowest) level will be highlighted (enlarged) and thatmyotome name will be displayed, as well as graphically depicted on thespine diagram. A vertical bar chart will also be shown, to depict thestimulation current required for nerve depolarization in mA for theselected channel. The screw test algorithm preferably determines thedepolarization (threshold) current for all 8 EMG channels. The surgeonmay also set a baseline threshold current by stimulating a nerve rootdirectly with the screw test probe 32. The surgeon may choose to displaythe screw test threshold current relative to this baseline. The handle44 may be equipped with a mechanism (via hardware and/or software) toidentify itself to the system when it is attached. In one embodiment,the probe member 46 is disposable and the handle 44 is reusable andautoclavable.

An audio pick-up (not shown) may also be provided as an optional featureaccording to the present invention. In some cases, when a nerve isstretched or compressed, it will emit a burst or train of spontaneousnerve activity. The audio pick-up is capable of transmitting soundsrepresentative of such activity such that the surgeon can monitor thisresponse on audio to help him determine if there has been stress to thenerve.

Analysis of the EMG responses according to the present invention willnow be described. The nerve proximity, nerve direction, and nervepathology features of the present invention are based on assessing theevoked response of the various muscle myotomes monitored by the surgicalsystem 10. This is best shown in FIGS. 3-4, wherein FIG. 3 illustratesthe evoked response (EMG) of a monitored myotome to the stimulationcurrent pulse shown in FIG. 4. The EMG response can be characterized bya peak to peak voltage of V_(pp)=V_(max)−V_(min). The stimulationcurrent is preferably DC coupled and comprised of monophasic pulses of200 microsecond duration with frequency and amplitude that is adjustedby the software. For each nerve and myotome there is a characteristicdelay from the stimulation current pulse to the EMG response.

As shown in FIG. 5, there is a threshold stimulation current required todepolarize the main nerve trunk. Below this threshold, currentstimulation does not evoke a significant V_(pp) response. Once thestimulation threshold is reached, the evoked response is reproducibleand increases with increasing stimulation, as shown in FIG. 5. This isknown as a “recruitment curve.” In one embodiment, a significant Vpp isdefined to be a minimum of 100 uV. The lowest stimulation current thatevoked this threshold voltage is called I_(thresh). I_(thresh) decreasesas the stimulation electrode approaches the nerve. This value is usefulto surgeons because it provides a relative indication of distance(proximity) from the electrode to the nerve.

As shown in FIG. 6, for each nerve/myotome combination there is acharacteristic delay from the stimulation current pulse to the EMGresponse. For each stimulation current pulse, the time from the currentpulse to the first max/min is T₁ and to the second max/min is T₂. Thefirst phase of the pulse may be positive or negative. As will bedescribed below, the values of T₁,T₂ are each compiled into a histogramwith bins as wide as the sampling rate. New values of T₁, T₂ areacquired with each stimulation and the histograms are continuouslyupdated. The value of T₁ and T₂ used is the center value of the largestbin in the histogram. The values of T₁, T₂ are continuously updated asthe histograms change. Initially Vpp is acquired using a window thatcontains the entire EMG response. After 20 samples, the use of T₁, T₂windows is phased in over a period of 200 samples. Vmax and Vmin arethen acquired only during windows centered around T₁, T₂ with widths of,by way of example only, 5 msec. This method of acquiring V_(pp) isadvantageous in that it automatically performs artifact rejection (aswill be described in greater detail below).

As will be explained in greater detail below, the use of the“recruitment curve” according to the present invention is advantageousin that it provides a great amount of useful data from which to makevarious assessments (including, but not limited to, nerve detection,nerve direction, and nerve pathology monitoring). Moreover, it providesthe ability to present simplified yet meaningful data to the user, asopposed to the actual EMG waveforms that are displayed to the users intraditional EMG systems. Due to the complexity in interpreting EMGwaveforms, such prior art systems typically require an additional personspecifically trained in such matters. This, in turn, can bedisadvantageous in that it translates into extra expense (having yetanother highly trained person in attendance) and oftentimes presentsscheduling challenges because most hospitals do not retain suchpersonnel. To account for the possibility that certain individuals willwant to see the actual EMG waveforms, the surgical system 10 includes anEvoked Potentials display that shows the voltage waveform for all 8 EMGchannels in real time. It shows the response of each monitored myotometo a current stimulation pulse. The display is updated each time thereis a stimulation pulse. The Evoked Potentials display may be accessedduring Detection, Direction, or Nerve Pathology Monitoring.

Nerve Detection (Proximity)

The Nerve Detection function of the present invention is used to detecta nerve with a stimulation electrode (i.e. those found on the surgicalaccess components 24-28) and to give the user a relative indication ofthe proximity of the nerve to the electrode are advanced toward thesurgical target site. A method of nerve proximity detection accordingone embodiment of the present invention is summarized as follows: (1)stimulation current pulses are emitted from the electrode with a fixedpulse width of 200 μs and a variable amplitude; (2) the EMG response ofthe associated muscle group is measured; (3) the Vpp of the EMG responseis determined using T1, T2, and Fmax (NB: before T2 is determined, aconstant Fsafe is used for Fmax); (4) a rapid hunting detectionalgorithm is used to determine I_(Thresh) for a known Vthresh minimum;(5) the value of I_(t) is displayed to the user as a relative indicationof the proximity of the nerve, wherein the I_(Thresh) is expected todecrease as the probe gets closer to the nerve. A detailed descriptionof the algorithms associated with the foregoing steps will follow aftera general description of the manner in which this proximity informationis communicated to the user.

The Detection Function displays the value of I_(thresh) to the surgeonalong with a color code so that the surgeon may use this information toavoid contact with neural tissues. This is shown generally in FIG. 7,which illustrates an exemplary screen display according to the presentinvention. Detection display is based on the amplitude of the current(I_(thresh)) required to evoke an EMG Vpp response greater thanV_(thresh) (nominally 100 uV). According to one embodiment, ifI_(thresh) is <=4 mA red is displayed, the absolute value of I_(thresh)displayed. If 4 mA<I_(thresh)<10 mA yellow is displayed. IfI_(thresh)>=10 mA green is displayed. Normally, I_(thresh) is onlydisplayed when it is in the red range. However, the surgeon has theoption of displaying I_(thresh) for all three ranges (red, yellow,green). The maximum stimulation current is preferably set by the userand is preferably within the range of between 0-100 mA. Detection isperformed on all 4 channels of the selected side. EMG channels on theopposite side are not used. The first dilator 26 may use an obturatorhaving an electrode for stimulation. In one embodiment, all subsequentdilators 26 and the working cannula 28 use four electrodes forstimulation. The lowest value of I_(thresh) from the 4 electrodes isused for display. There is an “Advance/Hold” display that tells thesurgeon when the calculations are finished and he may continue toadvance the instrument.

The threshold-hunting algorithm employs a series of monopolarstimulations to determine the stimulation current threshold for each EMGchannel that is in scope. The nerve is stimulated using current pulseswith amplitude of Istim. The muscle groups respond with an evokedpotential that has a peak to peak voltage of Vpp. The object of thisalgorithm is to quickly find I_(Thresh). This is the minimum Istim thatresults in a Vpp that is greater than a known threshold voltage Vthresh.The value of Istim is adjusted by a bracketing method as follows. Thefirst bracket is 0.2 mA and 0.3 mA. If the Vpp corresponding to both ofthese stimulation currents is lower than Vthresh, then the bracket sizeis doubled to 0.2 mA and 0.4 mA. This exponential doubling of thebracket size continues until the upper end of the bracket results in aVpp that is above Vthresh. The size of the brackets is then reduced by abisection method. A current stimulation value at the midpoint of thebracket is used and if this results in a Vpp that is above Vthresh, thenthe lower half becomes the new bracket. Likewise, if the midpoint Vpp isbelow Vthresh then the upper half becomes the new bracket. Thisbisection method is used until the bracket size has been reduced to IresmA. I_(Thresh) is the value of Istim that is the higher end of thebracket.

More specifically, with reference to FIGS. 8A-8E, the threshold huntingwill support three states: bracketing, bisection, and monitoring. Astimulation current bracket is a range of stimulation currents thatbracket the stimulation current threshold I_(Thresh). The upper and/orlower boundaries of a bracket may be indeterminate. The width of abracket is the upper boundary value minus the lower boundary value. Ifthe stimulation current threshold I_(Thresh) of a channel exceeds themaximum stimulation current, that threshold is considered out-of-range.During the bracketing state, threshold hunting will employ the methodbelow to select stimulation currents and identify stimulation currentbrackets for each EMG channel in scope.

The method for finding the minimum stimulation current uses the methodsof bracketing and bisection. The “root” is identified for a functionthat has the value −1 for stimulation currents that do not evokeadequate response; the function has the value +1 for stimulationcurrents that evoke a response. The root occurs when the function jumpsfrom −1 to +1 as stimulation current is increased: the function neverhas the value of precisely zero. The root will not be known precisely,but only with some level of accuracy. The root is found by identifying arange that must contain the root. The upper bound of this range is thelowest stimulation current I_(Thresh) where the function returns thevalue +1, i.e. the minimum stimulation current that evokes response.

The proximity function begins by adjusting the stimulation current untilthe root is bracketed (FIG. 8B). The initial bracketing range may beprovided in any number of suitable ranges. In one embodiment, theinitial bracketing range is 0.2 to 0.3 mA. If the upper stimulationcurrent does not evoke a response, the upper end of the range should beincreased. The range scale factor is 2. The stimulation current shouldnever be increased by more than 10 mA in one iteration. The stimulationcurrent should never exceed the programmed maximum stimulation current.For each stimulation, the algorithm will examine the response of eachactive channel to determine whether it falls within that bracket. Oncethe stimulation current threshold of each channel has been bracketed,the algorithm transitions to the bisection state.

During the bisection state (FIGS. 8C and 8D), threshold hunting willemploy the method described below to select stimulation currents andnarrow the bracket to a width of 0.1 mA for each EMG channel with anin-range threshold. After the minimum stimulation current has beenbracketed (FIG. 8B), the range containing the root is refined until theroot is known with a specified accuracy. The bisection method is used torefine the range containing the root. In one embodiment, the root shouldbe found to a precision of 0.1 mA. During the bisection method, thestimulation current at the midpoint of the bracket is used. If thestimulation evokes a response, the bracket shrinks to the lower half ofthe previous range. If the stimulation fails to evoke a response, thebracket shrinks to the upper half of the previous range. The proximityalgorithm is locked on the electrode position when the responsethreshold is bracketed by stimulation currents separated by 0.1 mA. Theprocess is repeated for each of the active channels until all thresholdsare precisely known. At that time, the algorithm enters the monitoringstate.

During the monitoring state (FIG. 8E), threshold hunting will employ themethod described below to select stimulation currents and identifywhether stimulation current thresholds are changing. In the monitoringstate, the stimulation current level is decremented or incremented by0.1 mA, depending on the response of a specific channel. If thethreshold has not changed then the lower end of the bracket should notevoke a response, while the upper end of the bracket should. If eitherof these conditions fail, the bracket is adjusted accordingly. Theprocess is repeated for each of the active channels to continue toassure that each threshold is bracketed. If stimulations fail to evokethe expected response three times in a row, then the algorithmtransitions back to the bracketing state in order to reestablish thebracket.

When it is necessary to determine the stimulation current thresholds(I_(t)) for more than one channel, they will be obtained bytime-multiplexing the threshold-hunting algorithm as shown in FIG. 9.During the bracketing state, the algorithm will start with a stimulationcurrent bracket of 0.2 mA and increase the size of the bracketexponentially. With each bracket, the algorithm will measure the Vpp ofall channels to determine which bracket they fall into. After this firstpass, the algorithm will know which exponential bracket contains theI_(t) for each channel. Next, during the bisection state, the algorithmwill start with the lowest exponential bracket that contains an I_(t)and bisect it until I_(t) is found within 0.1 mA. If there are more thanone I_(t) within an exponential bracket, they will be separated outduring the bisection process, and the one with the lowest value will befound first. During the monitoring state, the algorithm will monitor theupper and lower boundaries of the brackets for each I_(t), starting withthe lowest. If the I_(t) for one or more channels is not found in it'sbracket, then the algorithm goes back to the bracketing state tore-establish the bracket for those channels.

The method of performing automatic artifact rejection according to thepresent invention will now be described. As noted above, acquiringV_(pp) according to the present invention (based on T1,T2 shown in FIG.6) is advantageous in that, among other reasons, it automaticallyperforms artifact rejection. The nerve is stimulated using a series ofcurrent pulses above the stimulation threshold. The muscle groupsrespond with an evoked potential that has a peak to peak voltage of Vpp.For each EMG response pulse, T1 is the time is measured from thestimulus pulse to the first extremum (Vmax or Vmin) T2 is the timemeasured from the current pulse to the second extremum (Vmax or Vmin).

The values of T1 and T2 are each compiled into a histogram with Tbinmsec bin widths. The value of T1 and T2 used for artifact rejection isthe center value of the largest bin in the histogram. To rejectartifacts when acquiring the EMG response, Vmax and Vmin are acquiredonly during windows that are T1±Twin and T2±Twin. Again, with referenceto FIG. 6, Vpp is Vmax-Vmin.

The method of automatic artifact rejection is further explained withreference to FIG. 10. While the threshold hunting algorithm is active,after each stimulation, the following steps are undertaken for each EMGsensor channel that is in scope: (1) the time sample values for thewaveform maximum and minimum (after stimulus artifact rejection) will beplaced into a histogram; (2) the histogram bin size will be the samegranularity as the sampling period; (3) the histogram will be emptiedeach time the threshold hunting algorithm is activated; (4) thehistogram will provide two peaks, or modes, defined as the two bins withthe largest counts; (5) the first mode is defined as T1; the second modeis defined as T2; (6) a (possibly discontinuous) range of waveformsamples will be identified; (7) for the first stimulation after thethreshold hunting algorithm is activated, the range of samples will bethe entire waveform; (8) after a specified number of stimulations, therange of samples will be limited to T1±0.5 ms and T2±0.5 ms; and (9)before the specified number of stimulations, either range may be used,subject to this restriction: the proportion of stimulations using theentire waveform will decrease from 100% to 0% (a sample of the curvegoverning this proportion is shown in FIG. 11). Peak-to-peak voltage(Vpp) will be measured either over the identified range of waveformsamples. The specified number of stimulations will preferably be between220 and 240.

According to another aspect of the present invention, the maximumfrequency of the stimulation pulses is automatically obtained withreference to FIG. 12. After each stimulation, Fmax will be computed as:Fmax=1/(T2+Safety Margin) for the largest value of T2 from each of theactive EMG channels. In one embodiment, the Safety Margin is 5 ms,although it is contemplated that this could be varied according to anynumber of suitable durations. Before the specified number ofstimulations, the stimulations will be performed at intervals of 100-120ms during the bracketing state, intervals of 200-240 ms during thebisection state, and intervals of 400-480 ms during the monitoringstate. After the specified number of stimulations, the stimulations willbe performed at the fastest interval practical (but no faster than Fmax)during the bracketing state, the fastest interval practical (but nofaster than Fmax/2) during the bisection state, and the fastest intervalpractical (but no faster than Fmax/4) during the monitoring state. Themaximum frequency used until F_(max) is calculated is preferably 10 Hz,although slower stimulation frequencies may be used during someacquisition algorithms. The value of F_(max) used is periodicallyupdated to ensure that it is still appropriate. This feature isrepresented graphically, by way of example only, in FIG. 12. Forphysiological reasons, the maximum frequency for stimulation will be seton a per-patient basis. Readings will be taken from all myotomes and theone with the slowest frequency (highest T2) will be recorded.

Nerve Direction

Once a nerve is detected using the working cannula 28 or dilatingcannulae 26, the surgeon may use the Direction Function to determine theangular direction to the nerve relative to a reference mark on theaccess components 24-28. This is also shown in FIG. 7 as the arrow Apointing to the direction of the nerve. This information helps thesurgeon avoid the nerve as he or she advances the cannula. The directionfrom the cannula to a selected nerve is estimated using the 4 orthogonalelectrodes on the tip of the dilating cannula 26 and working cannulae28. These electrodes are preferably scanned in a monopolar configuration(that is, using each of the 4 electrodes as the stimulation source). Thenerve's threshold current (I_(thresh)) is found for each of theelectrodes by measuring the muscle evoked potential response Vpp andcomparing it to a known threshold Vthresh. This algorithm is used todetermine the direction from a stimulation electrode to a nerve.

As shown in FIG. 13, the four (4) electrodes are placed on the x and yaxes of a two dimensional coordinate system at radius R from the origin.A vector is drawn from the origin along the axis corresponding to eachelectrode that has a length equal to I_(Thresh) for that electrode. Thevector from the origin to a direction pointing toward the nerve is thencomputed. This algorithm employs the T1/T2 algorithm discussed abovewith reference to FIG. 6. Using the geometry shown in FIG. 10, the (x,y)coordinates of the nerve, taken as a single point, can be determined asa function of the distance from the nerve to each of four electrodes.This can be expressly mathematically as follows:

-   -   Where the “circles” denote the position of the electrode        respective to the origin or center of the cannula and the        “octagon” denotes the position of a nerve, and d₁, d₂, d₃, and        d₄ denote the distance between the nerve and electrodes 1-4        respectively, it can be shown that:

$x = {{\frac{d_{1}^{2} - d_{3}^{2}}{{- 4}\; R}\mspace{14mu} {and}\mspace{14mu} y} = \frac{d_{2}^{2} - d_{4}^{2}}{{- 4}\; R}}$

-   -   Where R is the cannula radius, standardized to 1, since angles        and not absolute values are measured.

After conversion from (x,y) to polar coordinates (r,θ), then θ is theangular direction to the nerve. This angular direction is then displayedto the user as shown in FIG. 7, by way of example only, as arrow Apointing towards the nerve. In this fashion, the surgeon can activelyavoid the nerve, thereby increasing patient safety while accessing thesurgical target site. The surgeon may select any one of the 4 channelsavailable to perform the Direction Function. The surgeon shouldpreferably not move or rotate the instrument while using the DirectionFunction, but rather should return to the Detection Function to continueadvancing the instrument.

Insertion and advancement of the access instruments 24-28 should beperformed at a rate sufficiently slow to allow the surgical system 10 toprovide real-time indication of the presence of nerves that may lie inthe path of the tip. To facilitate this, the threshold currentI_(Thresh) may be displayed such that it will indicate when thecomputation is finished and the data is accurate. For example, when thedetection information is up to date and the instrument such that it isnow ready to be advanced by the surgeon, it is contemplated to have thecolor display show up as saturated to communicate this fact to thesurgeon. During advancement of the instrument, if a channel's colorrange changes from green to yellow, advancement should proceed moreslowly, with careful observation of the detection level. If the channelcolor stays yellow or turns green after further advancement, it is apossible indication that the instrument tip has passed, and is movingfarther away from the nerve. If after further advancement, however, thechannel color turns red, then it is a possible indication that theinstrument tip has moved closer to a nerve. At this point the displaywill show the value of the stimulation current threshold in mA. Furtheradvancement should be attempted only with extreme caution, whileobserving the threshold values, and only if the clinician deems it safe.If the clinician decides to advance the instrument tip further, anincrease in threshold value (e.g. from 3 mA to 4 mA) may indicate theInstrument tip has safely passed the nerve. It may also be an indicationthat the instrument tip has encountered and is compressing the nerve.The latter may be detected by listening for sporadic outbursts, or“pops”, of nerve activity on the free running EMG audio output (asmentioned above). If, upon further advancement of the instrument, thealarm level decreases (e.g., from 4 mA to 3 mA), then it is very likelythat the instrument tip is extremely close to the spinal nerve, and toavoid neural damage, extreme caution should be exercised during furthermanipulation of the Instrument. Under such circumstances, the decisionto withdraw, reposition, or otherwise maneuver the instrument is at thesole discretion of the clinician based upon available information andexperience. Further radiographic imaging may be deemed appropriate toestablish the best course of action.

Nerve Pathology

As noted above, the surgical system 10 accomplishes neural pathologymonitoring by electrically stimulating a retracted nerve root via one ormore stimulation electrodes at the distal end of the nerve rootretractor 30 while monitoring the EMG responses of the muscle groupinnervated by the particular nerve. FIG. 14 shows the differencesbetween a healthy nerve (A) and a pathologic or unhealthy nerve (B). Theinventors have found through experimentation that information regardingnerve pathology (or “health” of “status”) can be extracted from therecruitment curves generated according to the present invention (see,e.g., discussion accompanying FIGS. 3-5). In particular, it has beenfound that a health nerve or nerve bundle will produce a recruitmentcurve having a generally low threshold or “hanging point” (in terms ofboth the y-axis or Vpp value and the x-axis or I_(Stim) value), a linearregion having a relatively steep slope, and a relatively high saturationregion (similar to those shown on recruitment curve “A” in FIG. 14). Onthe contrary, a nerve or nerve bundle that is unhealthy or whosefunction is otherwise compromised or impaired (such as being impinged byspinal structures or by prolonged retraction) will produce recruitmentcurve having a generally higher threshold (again, in terms of both they-axis or Vpp value and the x-axis or I_(Stim) value), a linear regionof reduced slope, and a relatively low saturation region (similar tothose shown on recruitment curve “B” in FIG. 14). By recognizing thesecharacteristics, one can monitor nerve root being retracted during aprocedure to determine if its pathology or health is affected (i.e.negatively) by such retraction. Moreover, one can monitor a nerve rootthat has already been deemed pathologic or unhealthy before theprocedure (such as may be caused by being impinged by bony structures ora bulging annulus) to determine if its pathology or health is affected(i.e. positively) by the procedure.

The surgical system 10 and related methods have been described aboveaccording to one embodiment of the present invention. It will be readilyappreciated that various modifications may be undertaken, or certainsteps or algorithms omitted or substituted, without departing from thescope of the present invention. By way of example only, certain of thesealternate embodiments or methods will be described below.

a. Hanging Point Detection Via Linear Regression

As opposed to identifying the stimulation current threshold (I_(Thresh))based on a predetermined V_(Thresh) (such as described above and shownin FIG. 5), it is also within the scope of the present invention todetermine I_(Thresh) via linear regression. This may be accomplishedvia, by way of example only, the linear regression technique disclosedin commonly owned and co-pending U.S. patent application Ser. No.09/877,713, filed Jun. 8, 200 and entitled “Relative Nerve Movement andStatus Detection System and Methods,” the entire contents of which ishereby expressly incorporated by reference as if set forth in thisdisclosure in its entirety.

b. Hanging Point Detection Via Dynamic Sweep Subtraction

With reference to FIG. 15, the hanging point or threshold may also bedetermined by the following dynamic sweep subtraction method. The nerveis stimulated in step 80 using current pulses that increase from I_(Min)to I_(Max) (as described above). The resulting the neuromuscularresponse (evoked EMG) for the associated muscles group is acquired instep 82. The peak-to-peak voltage (Vpp) is then extracted in step 84 foreach current pulse according to the T1, T2 algorithm described abovewith reference to FIGS. 3-6. A first recruitment curve (S1) is thengenerated by plotting Vpp vs. I_(Stim) in step 86. The same nerve isthen stimulated such that, in step 88, the peak-to-peak voltage (Vpp)may be extracted by subtracting the V_(Max) from V_(Min) of each EMGresponse without the T1, T2 filters employed in step 84. A secondrecruitment curve (S2) is then generated in step 90 by plotting Vpp vs.I_(Stim). The generation of both recruitment curves S1, S2 continuesuntil the maximum stimulation current (I_(Max)) is reached (per thedecision step 92). If I_(Max) is not reached, the stimulation currentI_(Stim) is incremented in step 94. If I_(Max) is reached, then thefirst recruitment curve S1 is subtracted from the second recruitmentcurve S2 in step 96 to produce the curve “C” shown in step 98. Bysubtracting S1 from S2, the resulting curve “C” is actually the onsetportion of the recruitment curve (that is, the portion before thethreshold is reached) for that particular nerve. In this fashion, thelast point in the curve “C” is the point with the greatest value ofI_(Stim) and hence the hanging point.

c. Peripheral Nerve Pathology Monitoring

Similar to the nerve pathology monitoring scheme described above, thepresent invention also contemplates the use of one or more electrodesdisposed along a portion or portions of an instrument (including, butnot limited to, the access components 24-28 described above) for thepurpose of monitoring the change, if any, in peripheral nerves duringthe course of the procedure. In particular, this may be accomplished bydisposing one or more stimulation electrodes a certain distance from thedistal end of the instrument such that, in use, they will likely come incontact with a peripheral nerve. For example, a mid-shaft stimulationelectrode could be used to stimulate a peripheral nerve during theprocedure. In any such configuration, a recruitment curve may begenerated for the given peripheral nerve such that it can be assessed inthe same fashion as described above with regard to the nerve rootretractor, providing the same benefits of being able to tell if thecontact between the instrument and the nerve is causing pathologydegradation or if the procedure itself is helping to restore or improvethe health or status of the peripheral nerve.

d. Virtual Patient for Evoked Potential Simulation

With reference to FIG. 16, the present invention also contemplates theuse of a “virtual patient” device for simulating a naturally occurringrecruitment curve. This is advantageous in that it provides the abilityto test the various systems disclosed herein, which one would not beable to test without an animal and/or human subject. Based on thetypically high costs of obtaining laboratory and/or surgical time (bothin terms of human capital and overhead), eliminating the requirement ofperforming actual testing to obtain recruitment curves is a significantoffering. According to the present invention, this can be accomplishedby providing a device (not shown) having suitable software and/orhardware capable of producing the signal shown in FIG. 16. The devicewill preferably accept a sweeping current signal according to thepresent invention (that is, 200 microseconds width pulses sweeping inamplitude from 0-100 mA) and produce a voltage pulse having apeak-to-peak voltage (Vpp) that varies with the amplitude of the currentinput pulse. The relationship of the output Vpp and the inputstimulation current will produce a recruitment curve similar to thatshown. In one embodiment, the device includes various adjustments suchthat the features of the recruitment curve may be selectively modified.For example, the features capable of being modified may include, but arenot necessarily limited to, Vpp at onset, maximum stimulation current ofonselt (hanging point), the slope of the linear region and/or the Vpp ofthe saturation region.

While this invention has been described in terms of a best mode forachieving this invention's objectives, it will be appreciated by thoseskilled in the art that variations may be accomplished in view of theseteachings without deviating from the spirit or scope of the presentinvention. For example, the present invention may be implemented usingany combination of computer programming software, firmware or hardware.As a preparatory step to practicing the invention or constructing anapparatus according to the invention, the computer programming code(whether software or firmware) according to the invention will typicallybe stored in one or more machine readable storage mediums such as fixed(hard) drives, diskettes, optical disks, magnetic tape, semiconductormemories such as ROMs, PROMs, etc., thereby making an article ofmanufacture in accordance with the invention. The article of manufacturecontaining the computer programming code is used by either executing thecode directly from the storage device, by copying the code from thestorage device into another storage device such as a hard disk, RAM,etc. or by transmitting the code on a network for remote execution. Ascan be envisioned by one of skill in the art, many differentcombinations of the above may be used and accordingly the presentinvention is not limited by the scope of the appended claims.

1. A method of establishing an operative corridor for spinal surgery,comprising: mounting a plurality of sensors proximate to selected legmuscles; activating a nerve monitoring system that outputs a stimulationsignal to one or more surgical access instruments and that, in responseto signals from the sensors, displays neuromuscular response informationwhich is indicative of nerve proximity; advancing an initial dilatingassembly through bodily tissue having neural structures in a selectedpath toward a targeted intervertebral disc of a spine, the initialdilating assembly comprising an initial dilator cannula and a removableinner element having at least one stimulation electrode that deliversthe stimulation signal to the bodily tissue prior to accessing thespine; during advancement of the initial dilating assembly through thebodily tissue and prior to accessing the spine, viewing theneuromuscular response information on a display device of the nervemonitoring system when the stimulation signal is delivered to the bodilytissue prior to accessing the spine; inserting a guide wire through alumen of the initial dilator cannula after the initial dilator cannulais advanced through the bodily tissue so that the guide wire penetratesan annulus of the targeted intervertebral disc of the spine; advancing aplurality of sequential dilator cannulas along the selected path towardthe targeted intervertebral disc of the spine after the guide wireengages the annulus of the disc; advancing a working corridor instrumentover an outermost dilator cannula of the plurality of sequential dilatorcannulas toward the targeted intervertebral disc of the spine after; andestablishing an operative corridor to the targeted intervertebral discof the spine using the working corridor instrument.
 2. The method ofclaim 1, wherein the removable inner element of the initial dilatingassembly comprises an obturator arranged inside the initial dilatorcannula and the at least one stimulation electrode.
 3. The method ofclaim 1, further comprising removing the removable inner element fromthe initial dilator cannula prior to inserting the guide wire throughthe initial dilator cannula.
 4. The method of claim 3, furthercomprising operating one or more buttons of a stimulator driver coupledto the removable inner element of the initial dilating assembly toselectively deliver the stimulation signal.
 5. The method of claim 1,further comprising inputting a setting for the stimulation signal usinga graphical user interface of the nerve monitoring system.
 6. The methodof claim 1, wherein the neuromuscular response information is astimulation current threshold required to evoke a neuromuscular responseof greater than or equal to a predetermined magnitude.
 7. The method ofclaim 6, wherein the nerve monitoring system contemporaneously displaysthe stimulation current threshold which is indicative of nerve proximityand a spine image indicative of sensor placement.
 8. The method of claim6, wherein the nerve monitoring system executes an algorithm todetermine the stimulation threshold that is displayed.
 9. The method ofclaim 1, wherein the sensors are EMG electrodes that detect voltageresponses from the selected muscles.
 10. The method of claim 9, whereinthe nerve monitoring system also displays EMG waveforms.
 11. The methodof claim 10, wherein the EMG waveforms displayed by the nerve monitoringsystem comprises the voltage waveforms for eight EMG response channels.12. The method of claim 1, further comprising removing the initialdilator cannula and the plurality of sequential dilator cannulas fromthe working corridor instrument prior to establishing the operativecorridor to the targeted intervertebral disc of the spine.
 13. Themethod of claim 1, wherein the working corridor instrument comprises acannula that defines an inner lumen.